1. Field of the Invention
The present invention relates to a prepulse condition/heat shut off condition determination method and apparatus for optical recording, and an optical recording method and apparatus.
2. Related Background Art
At present, optical recording is achieved by exclusively utilizing a thermal nature of a laser beam, and recording media (optical disks) include (1) a write-once type optical disk (pit formation type) allowing recording only once, such as an optical disk having a thin metallic film or thermet film as a recording layer, and (2) an optical disk which allows repeated recording, reproduction, and erasure, such as a magnetooptical disk having a magnetic thin film as a recording layer, a phase-change optical disk having a metallic film or thermet film as a recording layer, which film causes a phase change between crystal and amorphous phases, and the like.
Several ten thousands of tracks on which information is to be recorded are spirally or concentrically formed on an optical disk. Two types of information units corresponding to "0" and "1" are formed on each track, thereby recording information. In practice, the track itself (i.e., a background portion) indicates a first information unit corresponding to one of "0" and "1", and second information units (called marks, recently) corresponding to the other one of "0" and "1" are formed on the track in a point or island pattern. In this case, the presence/absence of marks, the mark interval, the mark length, the mark formation start position (i.e., the leading edge position of a mark), the mark formation end position (i.e., the trailing edge position of a mark), and the like express information. In particular, a method of expressing information by the edge position of a mark is called mark length recording.
An optical recording apparatus is mainly constituted by a laser source, a radiation optical system for radiating a laser beam emitted from the laser source onto an optical disk, modulation means for modulating the laser beam intensity according to information to be recorded, and optical disk rotation means. In a magnetooptical recording apparatus, magnetic means for applying a bias magnetic field to the radiation position of the beam is added.
The method for performing recording on an optical disk based on mark length recording includes a digital recording method and an analog recording method.
In general, digital recording is a method of performing recording using several types of fixed length marks determined by a modulation method. For example, when one seven RLL (Run Length Limited) modulation coding is used, the number of types of marks is 7 (2T to 8T; T is the write clock).
In contrast, in analog recording, mark lengths and mark intervals are arbitrary. The analog recording is mainly used for recording picture and sound signals, and is performed in the following process. First, a picture carrier obtained by frequency-modulating a picture signal and a sound carrier obtained by frequency-modulating a sound signal are added at a predetermined amplitude ratio. The synthesized signal is binarized by a certain threshold level to be converted into two types of information units corresponding to "0" and "1". Either of "0" or "1" is recorded on an optical disk as a mark. This recording process is shown in FIG. 12. In this case, the mark length represents information of the picture signal, and the duty (the ratio of the mark length to the mark period) represents information of the sound signal.
The same principle of optical recording applies to both the digital and analog methods. The principle will be described below.
Since optical recording exclusively utilizes a thermal nature of a laser beam (heat mode), the laser beam intensity need only be pulse-modulated between a relatively high first level and a relatively low base level (second level) in principle. When the laser beam intensity is at the first level, a mark is formed; when it is at the second level, no mark is formed. That is, one mark is formed in correspondence with one pulse. The second level can be zero since it does not form any mark. However, when a mark is to be formed, in other words, when the leading edge of a mark is to be formed, it is preferable that the disk temperature state immediately before formation be always positively maintained in a constant temperature state. Otherwise, the leading edge position varies depending on the temperature state immediately before formation. Such a variation disturbs high-density recording. Thus, it is preferable that an optical disk be pre-heated to a predetermined temperature .THETA..sub.pre, i.e., be set in a pre-heat state, and the second level be normally set at an intensity P.sub.pre for maintaining this pre-heat state (temperature .THETA..sub.pre). The temperature .THETA..sub.pre allows the disk temperature immediately before mark formation to be constant independently of the peak temperature position of the beam or the data pattern recorded at the spot center position, and P.sub.pre is given by the following formula: EQU .THETA..sub.pre =A.times.P.sub.pre .times.{1-exp(-.infin./.tau.)}+.THETA.A formula (3)
where A (.degree.C./mW) is the heat efficiency of the laser beam intensity determined by the disk, the spot, and the recording line density, and .THETA.A (.degree.C.) is the disk temperature in a non-radiation state of the beam.
The first mark formation is a method of forming one mark in correspondence with one pulse. FIG. 8 is a waveform chart of the laser beam intensity when one mark is formed by the first method. As shown in FIG. 8, a pulse waveform for raising the laser beam intensity from the base level (second level) P.sub.pre to start mark formation, and after the raised intensity (first level) P.sub.W1 is maintained for a time T.sub.W1 by a half-width, reducing the intensity to P.sub.pre is used. In this case, when the mark length is large, an adverse effect due to heat accumulation appears. The adverse effect is that even when the laser beam intensity is reduced to P.sub.pre to end mark formation, the medium temperature cannot be easily decreased to the mark formation start temperature or less due to the heat accumulation so far. For this reason, the mark length or width becomes undesirably large. This adverse effect is called "recording data pattern dependency of the mark formation end position, i.e., the mark trailing edge position". This dependency disturbs high-density recording.
The second mark formation method can solve this problem to some extent. FIG. 9 is a waveform chart of the laser beam intensity when one mark is formed by the second method. In this method, in order to form a mark, the intensity of the laser beam is raised from P.sub.pre to an intensity P.sub.W1 higher than P.sub.pre, the intensity is reduced to an intensity P.sub.W2 lower than P.sub.W1 after P.sub.W1 is maintained for a time T.sub.W1 by a half-width, and the intensity is reduced to P.sub.pre to end mark formation after P.sub.W2 is maintained for a predetermined period of time. This method is especially called a prepulse method.
In the prepulse method, P.sub.W1, T.sub.W1, and P.sub.W2 are called a prepulse condition. Conventionally, the prepulse condition is determined based on experience, and is fixed irrespective of the types of optical disks and individual disks.
Under the conventional fixed prepulse condition, it is difficult to minimize "recording data pattern dependency of the mark trailing edge position". More specifically, a mark has a teardrop shape having a larger trailing end portion, or conversely, has a teardrop shape having a larger leading end portion. Therefore, in the case of high-density recording, the trailing edge of the mark cannot be recorded at a precise position in some optical disks to be used. In particular, in the analog recording method using an arbitrary mark length, this problem is conspicuous. It is an object of the present invention to solve this problem.